custom color property in panel draw layout
The graphical representation is determined by the subtype field which is set in this example to 'COLOR'.

from bpy.props import FloatVectorProperty
...

object_color = FloatVectorProperty(  
   name="object_color",
   subtype='COLOR',
   default=(1.0, 1.0, 1.0),
   min=0.0, max=1.0,
   description="color picker"
   )



Documentation on custom properties,
Nice tutorial on addons